Developmental and Structural Section
McKown, Athena D. , Dengler, Nancy G. .
Acceleration of leaf vein pattern formation in C4 Flaveria (Asteraceae).
Leaf venation in many C4 species is characterized by high vein density, which is essential for C4 photosynthetic function by facilitating rapid intercellular diffusion of metabolites between tissues involved in C4 photosynthesis (mesophyll and bundle sheath). In this study, C3 Flaveria robusta and C4 F. bidentis were compared for whole leaf vein pattern in mature leaves and formation of vein pattern in developing leaves. Mature leaves did not show any substantial differences in major vein density between C3 and C4 Flaveria species, whereas C4 F. bidentis had greater minor vein density than C3 F. robusta. The developmental study showed that both major and minor vein patterning occurred early in leaf ontogeny in C4 species and at a comparable stage in development (based on the leaf plastochron index, LPI) as vein pattern formation in C3 species. Results showed that vein patterning observed in C4 F. bidentis was achieved through accelerated formation of minor veins in LPI -0.7 leaves. This occurred by further iteration of the general branching pattern observed in C3 F. robusta, rather than a prolongation in the timing of vein pattern formation. The expansion of bundle sheath tissue occurred after the establishment of vein pattern, and the maturation of continuous protoxylem in C4 F. bidentis. This supports the hypothesis that procambium formation is accelerated in C4 species relative to ground tissue development. In addition, mesophyll cells ceased dividing sooner and enlarged less in C4 F. bidentis than C3 F. robusta, thereby contributing to the higher vein density in C4 F. bidentis. Altered vein density is hypothesized as an early step in the evolution of C4 photosynthesis and may have been relatively easy to evolve (e.g. through a change in auxin production). This may establish a “precondition” facilitating the evolution of C3-C4 intermediacy and C4 photosynthesis.
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1 - University of Toronto, Department of Botany, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada
Presentation Type: Oral Paper:Papers for Sections
Location: Boulevard C/Hilton
Date: Tuesday, July 10th, 2007
Time: 4:30 PM